You have a good 10+ points of armor penetration from runes right? Add another +6 from mastery = 16.

By the time you get last whisper, they might have what? 30 armor? Maybe 40 on the outside?

So you penetrate a whopping 40% of the 15 to 25 they have left. Thats 6 to 10 points of armor penetration. Which is equal to what? Maybe 100 to 200 gold worth of penetration. It doesn't do much and will never bring them to 0 armor.

On the other hand, a Brutalizer adds straight armor penetration. This would ensure that you bring any 35 or less armor champ to ZERO which is king vs casters. Ok, maybe a bad example due to lack of attack speed.

But consider sword of the divine. Massive 60% Atk speed, plus a no-dodge and +30 Armor pierce clicky. That seems extremely good to me. What would u think about htis approach?